The present disclosure relates generally to apparatus and methods for image sensing, and, more particularly, to a color filter array for an image sensor, and to an image sensor comprising the color filter array, the color filter array having a pattern well-suited for mitigating the effects of cross-talk.
In the world of image sensors, increasing pixel count while decreasing pixel size has been an important trend that has seen pixel pitch reduced to 1 μm and below and pixel count increase to over 40 million pixels. This drive to decrease pixel size poses a number of significant challenges. A fundamental challenge is the reduction in light collection that has been mitigated by a variety of approaches including the use of microlenses and backside illumination (BSI).
Another problem that remains persistent in small pixels is the increased occurrence of cross talk. Cross talk may occur in two different ways. First, light incident above one pixel may penetrate into a neighboring pixel and generate photocharge. For example, a photon incident at some off-vertical (e.g., obtuse) angle may pass through a color filter (a red photon going through a red filter) overlying a first pixel, but due to the angle the red photon crosses over into and is absorbed by a neighboring pixel instead. Since that pixel might be covered by a green filter, the red photon erroneously contributes to the green signal. This is known as optical cross talk and tends to be very important in frontside illuminated (FSI) pixels. As pixel sizes decrease to levels comparable to the wavelength of the visible light, increased diffraction increases this form of cross talk in both FSI and BSI pixels.
In the second cross-talk mechanism, the charge generated in a pixel diffuses into neighboring pixels and contributes to the wrong signal. This is known as electrical or diffusion cross talk. Decreasing pixel sizes shortens the length over which the charge has to diffuse to reach neighboring pixels, aggravating the impact.
Cross talk in color image sensor pixels diminishes the color signal of affected color channels and increases the overlap in the spectral responses of the different color channels. For instance, in the Bayer pattern, the cross talk in the red pixel extends its spectral response into the green wavelength region and decreases the response in the red spectral region. The diminished color signal as a result of cross talk reduces the color gamut that can be reproduced from the raw color signal without color correction.
Typically, the standard sRGB color gamut can be reproduced by means of color correction. If, however, the cross talk substantially diminishes the color gamut of the device, more intensive color correction is required. The color correction must perform an amplification operation to transform the reduced gamut, and increased signal subtraction is required to compensate for the increased overlap in spectral responses. Increased cross talk therefore increases the noise amplification of the color correction process and leads to reduced signal-to-noise ratio (SNR) performance. Color correction matrices for sensors with increased cross talk will therefore sacrifice either color reproduction accuracy or SNR, or both.
Several approaches have been suggested both for reducing the effects of cross talk and for mitigating its impact on SNR and color reproduction accuracy. Most approaches have been centered on the idea of modifying the pixel structure to minimize cross-pixel light absorption. For instance, M. Furumiya et al., “High sensitivity and no-cross-talk pixel technology for embedded CMOS image sensor,” IEEE Trans. Electron Devices 48, 2221-2227 (2001), employs a double light shield to suppress cross talk caused by obliquely incident light. And US Patent Application Publication No. 2012/0025199 to S.-Y. Chen et al. uses deep trench isolation to reduce electrical cross talk between pixels. More recently, Samsung's ISOCELL pixel technology has targeted cross-talk reduction by introducing an insulating layer between pixels to prevent electrical cross talk and lower cross talk occurrence by a reported 30%. See, e.g., Samsung Tomorrow, “Samsung launches ISOCELL: Innovative image sensor technology for premium mobile devices,” http://global.samsungtomorrow.com/?p=28442.
Other approaches modify the color filter array design specifically to mitigate the occurrence and effect of cross talk. See, e.g., U.S. Pat. No. 8,054,352 to J. Kim and H. Tanaka; and US Patent Application Publication No. 2012/0019695 to Y. Qian et al. More specifically, for example, in the approach described in U.S. Pat. No. 8,054,352, the cross-talk behavior of the pixel is first characterized, and then a transformation is determined to map the spectral response in the presence of cross talk to the desired spectral response. This transformation is implemented as an adjustment in the composition of pigments in the color filters.